Ignitron excitation control circuit



IGNITRON EXCITATION CONTROL CIRCUIT Filed March 25, 1963 "I C 0 F IE. E

E 0 INVENTOR. m ERNEST G. ANGER United States Patent 3,2 29,161 IGNITRONEXCITATIGN CONTROL CIRCUlT Ernest G. Anger, Wauwatosa, Wis., assignor toSquare D Company, Paris Ridge, Ill., a corporation of Michigan FiledMar. 25, 1963, Ser. No. 267,462 14 Claims. (Cl. 315-196) This inventionrelates to the control of electric discharge devices and is moreparticularly concerned with a system for controlling the ignitioncurrent through the ignition electrode of mercury pool cathode-typeelectronic discharge devices commonly known as ignitrons.

Ignitron tubes are frequently used to control the magnitude ofalternating currents to electrical loads which require frequentswitching. While the control which will hereinafter be described may beused in various electrical systems, it is particularly suited for use ina resistance welder apparatus to control the flow of welding currentthrough the primary winding of the welding transformer.

An ignitron has a pool of liquid mercury, as a cathode, and is providedwith an ignition electrode known as the ignitor to initiate conductionof the ignitron. The ignitor may consist of a tapered piece of boroncarbide that extends down into the mercury pool without being wet by themercury. \Vhen a sufiicient potential is impressed between the ignitorand cathode of an ignitron, a small are is developed. If the currentforming the arc has a suffi cient intensity, electrons are releasedwhich ionize the mercury vapor within the ignitron. If the anode of theignitron has a positive polarity relative to the cathode and sufiicientionized mercury vapor is present, the ignitron switches from anonconductive high impedance state to a conductive state presentingpractically no impedance between the anode and cathode.

In a welding apparatus the ignitrons are usually connected in inverseparallel pairs between an alternating current supply and the primarywinding of a welding transformer to supply alternate half cycles ofalternating current to the primary winding. A weld timer determines theconduction interval of the ignitrons by supplying an appropriate signalto a means for initiating conduction of the ignitrons at adjustableinstants during the half cycle interval when the anodes of the ignitronshave a positive polarity.

One of the most commonly used circuits for initiating the conduction ofignitrons is known as an anode firing circuit wherein a grid controlledgaseous electrode tube, such as a thyratron, is connected between theanode and control electrode of the ignitron. When this arrangement isemployed, current flow at the anode of the ignitron is conducted by thethyratron to the ignition electrode. Another form of control which maybe employed to initiate the conduction of the ignitron comprehends anarrangement whereby a separate source is connected through a thyratronacross the control electrode and cathode of the ignitron. In thisarrangement the separate source provides the exciting current toinitiate the conduction of the ignitron. The disadvantage of theseparate excitation arrangement is-that relatively large components arerequired and proper synchronization of the current flow through theignitor with the anode voltage of the ignitron is difiicult to control.A distinct advantage of the anode firing arrangement, other than the useof less expensive components, is that it inherently causes a continuedhow of ignition current until the ignitron becomes conductive, at whichtime the low impedance drop of the ignitron eifectively shorts out theexcitation circuit to the ignition electrode. Thus, as soon as theignitron becomes conductive, the ignition current ceases to flow andwill not cause premature failure of the ignition electrodes. It has also3,229,161 Patented Jan. 11, 1966 been found that anode firing, becauseof the lower energy requirements necessary for proper ignition of theignitrons, will cause the ignitrons to operate at a lower temperaturethan is possible with separate excitation firing. The circuit accordingto the present invention retains all of the advantages of an anodefiring circuit for ignitrons without incurring the disadvantagespresented by the thyratrons which have a relatively short life and thusrequire frequent replacement. The anode firing circuit for ignitronsaccording to the present invention uses static semiconductor devices,such as silicon controlled rectifiers, which are known to havepractically an indefinite life span when properly applied. The circuitis arranged so the silicon controlled rectifiers will not be subjectedto the full line voltage which is impressed across the ignitrons. Thiswill permit the silicon controlled rectifiers to be used in a circuitwhich is normally beyond their rated capacity to provide an ignitroncontactor which is low in cost as it can be assembled with standardcomponents and which will provide troublefree operation so as to reducethe maintenance cost and the high down-time costs which heretoforeoccurredin modern factories when an ignitron contactor is renderedinoperative because of a failure of a thyratron tube in the firingcircuit.

It is an object therefore of the present invention to provide a circuitfor initiating conduction of a pair of inversely connected-ignitronswhich includes a transformer means connected in circuit with the anodeand cathode of the ignitrons for supplying anode to cathode current to apair of unidirectional conducting devices.

Another object of the present invention is to provide an anode firingcircuit for a pair of inversely connected ignitrons, which firingcircuit is subjected to a substantially lower voltage than the loadcircuit of the ignitrons.

A further object of the present invention is to provide an anode firingcircuit for an ignitron which includes a semiconductor and a transformermeans which is connected in a circuit parallel with the ignitron and inseries with the semiconductor.

Another object of the present invention is to provide an anode firingcircuit for initiating the conduction of an ignitron which includes asilicon controlled rectifier and a transformer means which is shorted bythe ignitron when the ignitron is conductive and which is arranged tosupply the rectifier with current at a voltage substantially lower thanthe voltage of the supply for the ignitron.

A still further object of the present invention is to provide an anodefiring circuit for initiating conduction of a pair of ignitronsconnected in inverse parallel between an alternatng current supply and aprimary winding of a welding transformer in a resistance welderapparatus which circuit, in response to a signal from a weld timer ofthe welder apparatus, causes a pair of silicon controlled rectifiers tobecome conductive at adjust-able instants during-the half cycles aftervoltage zero of the voltage wave of the supply and wherein the siliconcontrolled re ct-ifiers are connected in series with the ignitor.electrode of the ignitrons and with a transformer means which isconnected in a circuit with the anodes of the .fir-ing circuit for apair of ignitrons incorporating the features of the present invention.

FIG. 2 illustrates a series of curves of one form of 3 the firing pulsewhich may be provided to cause conduction of the ignitrons in FIG. 1.

In the drawings, a firing circuit for an ignitron type contactor isillustrated which may be use-d to control the flow of current from analternating current source to an alternating current load. While thealternating current load shown is a resistance welding transformer inFIG. 1, it is understood that laternating current fiow to other typeloads may be controlled by the ignitron contactor employing the firingscheme according to my invention.

In FIG. 1, an alternating current source, not shown, is connected tosupply two sets of input leads L1 and L2. A weld timer indicated by thenumeral is provided to supply a suitable signal to initiate conductionof a pair of ignitrons 12 and 14 in a manner which will be hereinafterset forth. The weld timer shown will provide a suitable signal at thebeginning and during the weld interval to govern the flow of energy to apair of Welding electrodes 16 and 18 and may be any suitable well knowntype, such as the timer shown in Patent 3,015,067, issued December 26,1961, to C. W. Porter and assigned to the assignee of the presentinvention, or a timer as shown in application Serial No. 129,828, filedAugust 7, 1961, which has been assigned to the assignee of the presentinvention by the inventors C. F. Meyer and]. I. Eckl.

The timer shown in the Porter patent includes gaseous electronic tubesand the timer shown in the Meyer and Eckl application utilizes staticswitching devices, such as transistors, and is arranged to supplysignals as shown on curve C in FIG 2 of the drawings.

The weld timer 10 is energized from the source through the leads L1 andL2 and is connected through a pulsing transformer 20 to a signal pulser22. In FIG. 2, a curve A illustrates the voltage wave of the sourcewhich is connected to the leads L1 and L2. Because of the inductivecharacteristics of the welding transformer, the voltage of curve A willcause a lagging current flow in the welding transformer circuit as shownin curve B. The timer 10 is arranged at the beg-inning and during theweld interval to supply a signal as shown by curve C in FIG. 2; that is,a series of voltage pulses which occur as shown during the respectivehalf cycles of current shown by the curve B. It will be seen from curveC that at the beginning of the weld interval during periods of L1polarity, the output of the weld timer 10 will provide a negativepolarity voltage which exists throughout the current half cycle L1. Atthe beginning of the L2 half cycle, the output signal from the timer 10changes from a negative polarity voltage to a zero voltage. This signalcontinues throughout part of the L2 half cycle. The pulsing transformer20 is arranged so that if the voltage through its primary windingchanges it will provide an output signal consisting of sharp voltagespikes, as shown in curve D. The voltage output pulses of transformer 20are impressed as inputs to the signal pulser 22 which in responsethereto provides an output as shown on curve E which consists of aseries of sharp voltage spikes. The output of the signal pulser 22 isimpressed across a primary win-ding of a transformer 24.

If desired, the weld timer may be also provided with an arrangement forphase shifting the occurrence of the signals in FIG. 2. This arrangementwill provide heat control in the resistance welder and is achieved byshifting the occurrence of the signals of curve C in timed relation tothe source voltage as indicated by a broken line in curve C and acorresponding shift of the pulses in curves D and E which will cause theconduction of the ignitrons to be initiated at adjustable instants aftervoltage zero of the source voltage as shown by the broken line curve ofcurve B.

As shown in FIG. 1, the lead L1 is connected through a junction 25 to ananode 26 of ignitron 14 and through a junction 28 to a cathode 30 ofignitron 12. The lead L2 is connected through a primary winding 32 of awelding transformer to a junction 34 which is connected to an anode 36of the ignitron 12 and through a junction 38 to a cathode 40 of ignitron14. The primary winding 32 of the welding transformer supples energy toa secondary winding 42 which is connected to the welding electrodes 16and 18 to cause welding current to flow through a pair of parts 44 whichare to be welded together.

The ignitron 12 has an initiating elect-rode known as an ignitor 46 andthe ignitron 14 has an initiating electrode known as an ignitor 48. Theignitor 46 is connected through a junction 50 to a cathode electrode 52of a unidirectional conducting device, such as a silicon controlledrectifier 54, which has an anode 56 and a control electrode known as agate 58. The ignitor 48 of the ignitron 14 is connected through ajunction 60 to a cathode 62 of another unidirectional conducting device,such as a silicon controlled rectifier 64, which has an anode 66 andcontrol electrode, known as a gate 68.

A transformer winding 70 is connected between the junction 28 and 38. Inthe embodiment shown, the transformer winding 70 is provided with a pairof intermediate taps 72 and 74. The tap 72 is connected to the anode 56of the rectifier 54. The tap 74 is connected to the anode 66 of therectifier 64. Back-to-back rectifiers and 82 are connected as surgesuppressors between the anodes and the cathodes of the rectifiers 54 and64. These 'back-toback rectifiers 80 and 82 act as transient voltagesuppressors in the circuits associated therewith to protect therectifiers 54 and 64 from damage due to transient voltages.

The transformer 24 has a pair of secondary windings 84 and 86. One ofthe output terminals of the winding 84 is connected through a suitablediode 88 and a junction 90 to the gate 58 of the rectifier 54. The otheroutput terminal of the winding 84 is connected through a junction 92 tothe cathode 52 of the rectifier 54. A bias resistor 94 is connectedbetween junction-s 90 and 92. Similarly, one of the output terminals ofthe winding 86 is connected through a diode 96 and a junction 98 to thegate 68 of the rectifier 64. The other output terminal of the winding 86is connected through a junction 100 to the cathode 62 of the rectifier64. A resistor 102 is connected between junctions 98 and 100.

The operation of the circuit above described is as follows: if it beassumed that the ignitrons 12 and 14 are both nonconducting, currentwill flow through the winding 70 and the winding 32. The winding 70 isshown as connected as an auto-transformer and during intervals when thesecondary circuits connected to taps 72 and 74 are open, will provide ahigh impedance path for current flow through primary winding 32. Duringperiods when lead L1 is positive, the portion of the winding 70 betweenterminal 74 and junction 38 will cause the anode 66 of the rectifier 64to have a positive polarity relative to its cathode 62. Similarly, whenlead L2 is positive, the portion of the winding 70 between terminal 72and junction 28 will cause the anode 56 of the rectifier 54 to have apositive polarity relative to its cathode 52. As previously explained,when the ignitrons are nonconducting, they present a high impedancewhich changes to a low impedance as they change to their conductivestates. Thus, when either of the ignitrons 12 or 14 becomes conductive,a short circuit path is formed across winding 70 to remove the supplyfor the anode to cathode voltage of the rectifiers 54 and 64.

At the beginning and during the weld interval the transformer 24 causesthe windings 84 and 86 to be provided with a series of voltage pulseswhich occur as shown by curve B of FIG. 2. These voltage pulses areconducted by the diodes 88 and 96 to the junctions 90 and 98 where theyappear between the gates 58 and 68 and cathodes 52 and 62 of therectifiers 54 and 64 to cause the rectifiers to become conductive. Whenthe rectifier 54 conducts, current flows from the junction 72 throughthe rectifier 54 to the ignitor 46 to cause the ignitron 12 to conductas previously explained. Similarly, when the rectifier 64 is renderedconductive, current flows from the terminal 72 through'th'e rectifier 64to the ignitor 46 to cause the ignitron14 to be rendered conductive.

The winding 70 is preferably wound of a wire of suitahlesize to providelow resistance and the coil windings thereof are suitably interleaved toprovide low leakage reactance so the winding 70 provides a very lowshort circuit impedance at the terminals 72 and 74. This arrangementwill provide the system with a rapid response. The winding 70 as shownis connected in parallel with the ignitrons 12' and 14 so that as theignitrons 12 and 14 conduct, theyeffectively provide a short circuitacross the winding 70. While the winding 70 is shown as anautotransformer, other transformer arrangements may be used to redum theanode to cathode voltages of the rectifiers 54 and 64. The arrangementshown makes it possible to use silicon controlledrectifiers having alower voltage rating than would be required if the anodes of therectifiers were directly connected to the anodes of the ignitrons asconventional ignitron anode firing circuits. For example: if the sourcecauses the leads L1 and L2 to have a potential of 440 volts asis'conventional in resistance welding practice, then the rectifiers becauseof the use of the suitable taps of the winding 70, will provide an anodeto cathode voltage across the rectifiers of 110 volts which issufiicient to initiate the conduction of the ignitrons without damagingthe rectifiers or requiring special costly devices.

While certain preferred embodiments of the invention have beenspecifically disclosed, it is understood that the invention is notlimited thereto, as many variations will be readily apparent to thoseskilled in the art and the invention is to be given its broadestpossible interpretation within the terms of the following claims.

What is claimed is:

1. In combination with a source of alternating current having a pair ofterminals, a first ignitron having an anode, a cathode and an ignitionelectrode, a second ignitron having an anode, a cathode and an ignitionelectrode, means connecting the anode of the first ignitron and thecathode of the second ignitron to one terminal of the source, meansconnecting the cathode of the first ignitron and the anode of the secondignitron to the other terminal of the source, an autotransformer havinga winding interconnecting the anodes of both ignitrons, said transformerhaving at least one tap dividing said winding into two portions, a firstunidirectional conducting device having an anode, a cathode and acontrol electrode, a second unidirectional conducting device having ananode, a cathode and a control electrode, means connecting in seriesacross one of said winding portions the anode and the cathode of thefirst device and the ignition electrode and the cathode of the firstignitron, means connecting in series across the other of said windingportions the anode and the cathode of the second device and the ignitionelectrode and the cathode of the second ignitron, and circuit meansconnected between the control electrodes and the cathodes of each ofsaid devices for initiating conduction of said devices.

2. For use in supplying current from a source of alternating current toa load, the combination comprising: a pair of ignitrons interposed in acircuit between the source and the load in such a manner so as toconduct alternate half cycles of current from the source to the load,each of said ignitrons having an anode, a cathode and an ignitionelectrode arranged for switching the ignitrons from a nonconductive to aconductive state, transformer means connected in a circuit parallel withsaid ignitron circuit, said transformer having a pair of outputs, afirst unidirectional conducting device connected in a series with theignition electrode and the cathode of one of said ignitrons across oneof said outputs and a second unidirectional conducting device connectedin a series circuit with the 6 ignition electrode and the cathode of theother ignitron across the other of said outputs.

3. For use in supplying current from a source of alternating current toa load, the combination comprising; a pair of ignitrons interposed in afirst circuit between the source and the load in such a manner so as toconduct alternate half cycles of current from the source to the load,each of said ignitrons having an anode, a cathode and an ignitionelectrode arranged for switching the ignitrons from a nonconductive to aconductive state and thereby changing the impedance of said circuit froma high value to a negligible value, a pair of unidirectional conductingdevices each having an anode, a cathode and a control electrode arrangedfor initiating the conduction of the device, transformer meansinterposed in a circuit between the source and the load in a circuitparallel with said first circuit, said transformer means having animpedance substantially less than the high impedance value of said firstcircuit and greater than the negligible value of the impedance of saidfirst circuit when the ignitrons are conducting whereby the ignitronssubstantially provide a short circuit path for the transformer means,said transformer means having a pair of output portions, a secondcircuit means connecting the anode and the cathode of one of saiddevices in a series circuit with one of said output portions and theignition electrode and the cathode of the first ignitron and a thirdcircuit means connecting the anode and the cathode of the other of saidpair of devices in a series circuit with the other output portion andthe ignition electrode and the cathode of the second ignitron forinitiating conduction of the ignitrons in response to current flowthrough the transformer means when the ignitrons are nonconductive.

4. In combination; a pair of ignitrons connected in inverse parallelbetween an alternating current source and a load, each of said ignitronshaving an anode, a cathode and an ignition electrode, a pair ofunidirectional conducting devices each having an anode, a cathode and acontrol electrode for initiating conduction of said devices, circuitmeans connecting the anode and the cathode of one of said unidirectionalconducting devices in a series circuit with the ignition electrode andcathode of one of said ignitrons, circuit means connecting the anode andthe cathode of the other of said unidirectional conducting devices in aseries circuit with the ignition electrode and cathode of the other ofsaid ignitrons, and transformer means connected in a parallel circuitwith both of said ignitrons between said source and load and saidcircuit means including a portion of the transformer means in each ofsaid series circuits for initiating conduction of said ignitrons inresponse to the conduction of said unidirectional conducting devices.

5. The combination as set forth in claim 4 wherein each of theunidirectional conducting devices is a silicon controlled rectifier.

6. The combination as set forth in claim 4 wherein the transformer meanscomprises an autotransformer.

7. The combination as set forth in claim 4 wherein each of theunidirectional conducting devices is a silicon controlled rectifier andthe transformer means comprises an autotransformer.

8. The combination as set forth in claim 4 wherein each of theunidirectional conducting devices is a silicon controlled rectifierwhich has a firing circuit connected between the control electrode andthe cathode of the rectifier for initiating conduction of the rectifierand wherein the firing circuit includes means for providing a series ofpulses between the control electrode and cathode of the devices.

9. The combination as set forth in claim 8 wherein the ring circuit iscoupled to a weld timer for controlling the initiation of conduction ofthe devices and thereby the initiation of conduction of the ignitrons.

10. In combination; a pair of ignitrons connected in inverse parallelbetween an alternating current source and a load for supplying alternatehalf cycles of alternating current to the load, each of said ignitronshaving an anode, a cathode and an ignition electrode, a pair ofunidirectional conducting devices each having an anode, a cathode and acontrol electrode for initiating conduction of said devices, circuitmeans connecting the anode and the cathode of one of said unidirectionalconducting devices in a series circuit with the ignition electrode andthe cathode of one of said ignitrons, circuit means connecting the anodeand the cathode of the other of said unidirectional conducting devicesin a series circuit with the ignition electrode and the cathode of theother of said ignitrons, transformer means having a portion connected ina parallel circuit with both of said ignitrons between said source andload and a portion of said transformer means in each of said seriescircuits for initiating conduction of said ignitrons in response to theconduction of said devices, and firing means connecting between thecontrol electrode and cathode of each of said devices for adjustablyinitiating the conduction of the devices at variable instants during thehalf cycles of the supply.

11. The combination as set forth in claim 10 wherein the firing means isarranged to supply the devices with a series of current pulses after theinstant of initiation of conduction of the devices.

12. In combination: an ignitron having a pair of main electrodesconnected between an alternating current supply and a load, and an anodefiring circuit for the ignitron including a semiconductor and anautotransformer winding, said winding having a main portion connected tothe main electrodes of the ignitron to be in a circuit parallel to theignitron and said winding having a portion of said main portionconnected in a series circuit with pair of main electrodes of thesemiconductor.

13. The combination as recited in claim 12 wherein the semiconductor isa silicon controlled rectifier.

14. The combination as recited in claim 12lwherein the semi-conductorhas a control electrode which is arranged for initiating conduction ofthe semi-conductor connected in a circuit with a phase shift circuitmeans for initiating conduction of the semi-conductor at vari ableinstants of a half cycle current pulseof the alternating current supply.

References Cited by the Examiner UNITED STATES PATENTS 2,175,921 10/1939Schnarz 315-196 X 3,089,984 5/1963 Randolph 31S199 X 3,112,437 11/ 1963Rockafellow 323-24 JOHN W. HUCKERT, Primary Examiner. DAVID J. GALVIN,Examiner.

1. IN COMBINATION WITH A SOURCE OF ALTERNATING CURRENT HAVING A PAIR OFTERMINALS, A FIRST IGNITRON HAVING AN ANODE, A CATHODE AND AN IGNITIONELECTRODE, A SECOND IGNITRON HAVING AN ANODE, A CATHODE AND AN IGNITRONAND THE MEANS CONNECTING THE ANODE OF THE FIRST IGNITRON AND THE CATHODEOF THE SECOND IGNITRON TO ONE TERMINAL OF THE SOURCE, MEANS CONNECTINGTHE CATHODE OF THE FIRST IGNITRON AND THE ANODE OF THE SECOND IGNITRONTO THE OTHER TERMINAL OF THE SOURCE, AN AUTOTRANSFORMER HAVING A WINDINGINTERCONNECTING THE ANODES OF BOTH IGNITRONS, SAID TRANSFORMER HAVING ATLEAST ONE TAP DIVIDING SAID WINDING INTO TWO PORTIONS, A FIRSTUNIDIRECTIONAL CONDUCTING DEVICE HAVING AN ANODE, A CATHODE AND ACONTROL ELECTRODE, A SECOND UNIDIRECTIONAL CONDUCTING DEVICE HAVING ANANODE, A CATHODE AND A CONTROL ELECTRODE, MEANS CONNECTING IN SERIESACROSS ONE OF SAID WINDING PORTIONS THE ANODE AND THE CATHODE OF THEFIRST DEVICE AND THE IGNITION ELECTRODE AND THE CATHODE OF THE FIRSTIGNITRON, MEANS CONNECTING IN SERIES ACROSS THE OTHER OF SAID WINDINGPORTIONS THE ANODE AND THE CATHODE OF THE SECOND DEVICE AND THE IGNITIONELECTRODE AND THE CATHODE OF THE SECOND IGNITRON, AND CIRCUIT MEANSCONNECTED BETWEEN THE CONTROL ELECTRODES AND THE CATHODES OF EACH OFSAID DEVICES FOR INITIATING CONDUCTION OF SAID DEVICES.